Method of accentuating small differences in optical density for obtaining isodensity shapes which will provide quantitative comparisons

ABSTRACT

A method for obtaining isodensity lines from a continuous tone film which makes small differences in optical density discernible comprises exposing a negative photocopy emulsion to high intensity diffuse light of at least 800 foot candles through the film for a predetermined time, placing a positive emulsion in contact with the exposed negative, inserting them in a developing solution, removing them and separating them after a predetermined time, whereby all optical densities below a selected density, determined by the light exposure time, are erased from the positive and the image thereon is an isodensity outline of the image on the film in which lines of slightly varying density can be easily distinguished.

United States Patent [72] Inventor Charles Lescrenier 3,174,857 3/1965 Clarke 250/65 West Allis, Wis. (2209 E. Kenilworth Place, 2,426,884 9/1947 Kieffer 250/65 Milwaukee, Wis. 53202) OTHER REFERENCES [2]] P 583267 96 Standardization of Radiographic Technique, Weidman [221 F'led 30,1 6 et 8]., pages 213 to 226, 250-65 [45] Patented Feb. 16,1971

Primary Examiner-Archie R. Borchelt Assistant Examiner-A. L. Birch [54] METHOD OF ACCENTUATING SMALL Anorney- Lee H. Kaiser DIFFERENCES IN OPTICAL DENSITY FOR OBTAINING ISODENSITY SHAPES WHICH WILL VI E QUANTITATIVE MP R ABSTRACT: A method for obtaining isodensity lines from a 10 Claims, 8 Drawing Figs. continuous tone film which makes small differences in optical density discernible comprises exposing a negative photocopy [52] U.S.Cl. 250/65; emulsion to high intensity diffuse light ofat least 800 foot cam 355/132 355/133 dles through the film for a predetermined time, placing a posi- [50] new 4 9 tive emulsion in contact with the exposed negative, inserting 8 9 l 3 5/ 9 them in a developing solution, removing them and separating them after a predetermined time, whereby all optical densities [56] References and below a selected density, determined by the light exposure UNlTED STATES PATENTS time, are erased from the positive and the image thereon is an 2,835,167 5/l958 Pierce 350/19 isodensity outline of the image on the film in which lines of 3,l2l,792 2/1964 Mittelstaedt 250/65 slightly varying density can be easily distinguished.

EXPOSURE DEVELOPMENT HIGH CONTRAST COPY 1 1 1 a WORM 1 H GH INTENSITY LIGHT PATENTED FEBTBISYI' 3,564,242

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T UNlFORM HIGH INTENSITY LIGHT METHOD OF ACCENTUATING SMALL DIFFERENCES IN OPTICAL DENSITY FOR OBTAINING ISODENSITY SHAPES WHICH WILL PROVIDE QUANTITATIVE COMPARISONS This invention relates to a method of distinguishing optical densities on film which do not differ enough to be differentiated visually.

The analysis of information analogues such as radiographs, photographs, and other records on which information is stored is a continuous tone, two-dimensional, optical, density pattern can often be aided by reconstructing the record as an isodensity contour plot. Such an equal optical density line presents the information contained in the record in a directly useful, scaled and quantitative format and also improves the accuracy of photogrammetric measurements on objects with diffuse edges. Examplesof the use of isodensity plots for the study of radiation distribution in medical research and diagnosis include the production of contour lines of equal isotope takeup by body organs, production of lines of contrast of x-ray absorption for measurement of material densities or thicknesses, and study of morphological features of chromosomes by accentuating density patterns of enlarged micrographs.

Isodensity curves derived from radiographs have not been used extensively in the study of radiation distribution within tissues because of the expense and tedium associated with the extraction of isodensity curves from the film. The conventional method of isodensity plotting utilizes a precision densitometer which is scanned over the film to detect points of equal density, and this method is both time consuming and expensive.

It is an object of the invention to provide an improved method for distinguishing with high precision optical densities which are not visually discernible.

It is a further object of the invention to provide such an improved method of obtaining isodensity lines from photographic films which is substantially quicker, simpler, and less expensive than conventional densitometric scanning techniques.

Another object of the invention is to provide such an improved method for analyzing micrographs and autoradiographs with varying density distributions into isodensity lines which are capable of differentiation with greater precision between slightly different optical densities than prior art techniques and apparatus.

Still another object of the invention is to provide an improved method and apparatus for medical diagnosis of abnormal tissue growth such as tumors.

These and other objects of the invention will be more readily apparent from the following detailed description when considered in conjunction with the accompanying drawing wherein:

FIG. 1 illustrates a typical reproduction of a film exposed to radiation through tissue in which densities below a selected magnitude have been eliminated by the method of the invention;

FIG. 2 is a vertical section view through the photocopy machine utilized in practicing the invention;

FIG. 3 illustrates the step of placing the exposed negative photocopy emulsion, while held in contact with a positive emulsion, in a developer solution;

FIG. 4 illustrates an alternative method of practicing the invention wherein a negative photocopy emulsion is placed in contact with a positive emulsion on a transparent member;

FIG. 5 is an isodensity plot produced by an alternative mode of practicing the invention and showing x-ray absorption in a phantom;

FIG. 6 illustrates a step in the diagnosis of abnormal tissue growth utilizing the invention;

FIG. 7 is a calibration curve for the threshold density to be erased; and

FIG. 8 portrays the sequence of steps in practicing the invention.

The invention has numerous applications but will be described in relation to only a few representative uses; namely, the analysis of records on which information is stored in a continuous tone, two-dimensional, optical, density pattern, the diagnosis of abnormal tissue growth using radiation, the study of morphological features of chromosomes by accentuating density patterns on enlarged micrographs, the detection of slag or voids in industrial castings, and the comparison of roentgenographs on a quantitative rather than a qualitative basis without the necessity of controlling absolute densities.

The analysis of information analogues such as photographs, radiographs, and other photographic films on which informa' tion is stored in a continuous tone optical density pattern can often be aided by reconstructing the record as an isodensity plot. For example, an isodensity plot of equal isotope takeup by body organs from photographically recording gamma ray scanners or an isodensity plot of surface temperature from thermographic records is of considerable value in medical analysis and in radiation therapy. Such isodensity plots are conventionally obtained by the use of a precision microdensitometer to detect points of equal density on the photographic record, but the expense and tedium associated with the extraction of isodensity curves by such conventional techniques has prevented widespread use of such isodensity plots.

The invention quickly provides isodensity lines from photographic records at nominal expense by a diffusion transfer technique analogous to that used in photocopying. The method of the invention makes extremely small differences of density on a continuous tone photograph readily discernible in a manner which is much quicker, simpler, and less expensive than conventional densitometric scanning processes used to obtain isodensity patterns. Referring to FIG. 6, assume thata photographic film 1 has been exposed to radiation from an xray machine 2 passing through the tissue of a subject 4 to be studied and has been developed to provide an optical density pattern on the developed film representative of the x-ray absorption of the tissue of the subject 4. The developed film 10 (See FIG. 2) is placed in contact with the emulsion side of a negative photocopy paper 11 and laid on the transluscent plate 12 of a photocopy machine 14 having a hinged cover 15 and a bank of incandescent lamps 16 adapted to provide extremely high intensity light. For example, 52 lOO-watt light bulbs 16 may be utilized to provide an intensity of at least 800 and preferably 3000 foot candles of uniform light passing through translucent plate 12. It will be appreciated that light emanating from a source other than incandescent lamps can be utilized if the light source 16 provides uniform light of sufficiently high intensity to obtain the necessary contrast in the negative photocopy paper 11. The method of the invention utilizes the cutoff characteristic of the negative photocopy paper 11 which converts exposed silver halide particles to metalic silver in the developer and leaves no image on the final reproduction if a predetermined total quality of light reaches the particular point on the negative photocopy paper 11 during the exposure by light from incandescent lamps 16 passing through photographic film 10.

The exposed negative photocopy paper 11 is then removed from photocopy machine 14, separated from photographic film 10, placed in contact with a positive photocopy paper 17 (See FIGS. 3 and 8) so that the emulsion sides are together, and papers 11 and 17 so held in contact are then inserted in a developer solution 18. The contacting negative and positive papers 11 and 17 are left for a predetermined interval of time in the developer-solution l8, removed from the solution 18, and separated approximately l0 seconds after their removal from the developer. An image 20 shown in FIG. 1 remains on the positive paper 17 which outlines points of equal density on film 10 and from which all densities below a selected density value have been eliminated, this selected density value being determined by the exposure time in photocopy machine 14 as shown in FIG. 7. Light striking the silver halide particles of negative paper 11 affects them so that developer 18 is able to convert them to grains of metallic silver. The stronger the light acting upon the negative photocopy paper 11, the more discrete will be the light sensitive silver halide grains which are converted to metallic silver in the developer; and consequently, the greater will be the contrast in the final reproduction. Thus, in the bright parts of the image a large number of silver halide particles are affected, fewer in the half tones, and fewer still in the dark shadows. The high intensity light source 16 assures a high ratio of maximum to minimum metallic silver particles per unit area across the negative photocopy paper 11 when the emulsion side of the undeveloped negative photocopy paper 11 is placed in contact with the emulsion side of the positive photocopy paper 17 and left in developer solution 18. If a particular silver halide particle has not reached the threshold, by exposure to light, which will convert it to a metallic silver grain in the developer, the silver halide particle will diffuse to the positive photocopy paper 17 and react with and change the opposing silver halide particle in the positive paper 17 to a metallicsilver grain which appears as a dark spot on the positive paper 17. No marking will appear at a point on positive paper 17 after removal from the developer if a predetermined total quantity of light has reached the negative photocopy paper 11 during exposure in photocopying machine 14. The threshold density which will darken and produce an image on positive 17 can be varied at will as shown in FIG. 7 Differences in optical density as small as 0.005 can be visualized by my invention. The optical densities to be eliminated from the image on positive paper 17 can be precisely controlled by varying the exposure time of negative photocopy paper 11 in photocopy machine 14. In other words, the optical densities which are erased from the positive photocopy paper 17 are accurately determined in terms of exposure time of negative photocopy paper 11 in photocopy machine 14.

I have also found that if the emulsion side of exposed negative photocopy paper 11 is brought into contact with a positive photographic emulsion 22 carried on a transparent member 23 as illustrated in FIG. 4, if the contacting members 11 and 22 are inserted into the developer 18 for a predetermined length of time, and the contacting emulsions are not separated after being removed from the developer, an outline 24 visible through transparent member 23 appears around the selected density level as shown in FIG. which is an isodensity plot of x-ray absorption in a phantom. The thickness of the isodensity line 24 at any point is dependent upon the local density gradient. Normal positive paper can be used as an alternative to positive photographic emulsion 22 on transparent member 23, but in this case it is necessary to make a further copy of the unscparated positive and negative papers to obtain the desired isodensity line. It will be noted that this mode of practicing the invention provides a black-white-black isodensity plot as compared to the black-white isodensity curve shown in FIG. 1 proved by the preferred method.

The invention may also be used to distinguish differences in structures of matter which cannot be differentiated visually. For example, lung tissue may have different amounts of pigrnentation which prevent visual determination of the amount of real tissue. My invention permits reproduction of the structure of this tissue independent of such pigmentation. A thin slice of the lung tissue may be placed in contact with the emulsion side of the negative paper 11 in photocopy machine 14 and the negative paper 11 exposed to the intense light from source 16 passing through the slice of tissue. The exposed negative paper II is then removed from photocopy machine 14, separated from the slice of tissue, place in contact with a positive photocopy paper 17 so that the emulsion sides of the two papers are together, passed through a chemical developer 18 as shown in FIG. 8, and the positive and negative papers separated a predetermined time after they are removed from the developer. All densities below aselected density will be eliminated from the positive paper and the positive paper will bear an image which accentuates small differences in the density of the tissue which are indicative of the structure of the tissue.

My invention is advantageous in the detection of differences of thickness of matter due to absence of material supposed to be present, such as bubbles in glass and rubber and blow-holes and shrinkage areas in castings and is also useful in the detection of differences in composition due to the presence of foreign object supposed to be absent such as slag, inclusions in castings and welds or metal particles in insulation. For example, the size of a blow-hole in a metal casting may be determined in known manner by exposing a photographic emulsion to radiation passing through the casting and developing the emulsion. The outline and size of the void in the casting may be blurred and indistinguishable in the image on the developed radiographic'film 10. If the developed film 10 is then placed in contact with the emulsion side of a negative photocopy paper 11 and exposed to the uniform light from source 16 in photocopy machine 14 and the negative paper 11 processed in accordance with the steps described in detail hereinbefore, an image will remain on the positive 17 from which all densities below a selected density are erased, and

lines of slightly varying density, which cannot be distinguished visually from each, other on the radiograph 10 of the casting, can be readily distinguished from one another on the image on the developed positive paper 17. The outline of the void in the casting will be sharp in the image on positive paper 17 since the thickness of the isodensity lineis dependent upon the local density gradient in' the radiograph.

My invention is advantageous in the diagnosis of abnormal tissue growth such as tumors. It permits detection of tumors which cannot be found by known diagnostic means and is particularly advantageous in analyzing the growth of tumors over a period of time. For example,,bronchial cancer caused by smoking is fatal in an extremely high percentage of cases, and detection of bronchial cancer by comparison of radiographs of the chest of a patient taken at different times is very difficult. Theoretically, the patient should .be subjected to equal amounts of radiation to permit comparisons of the radiographs taken several months apart, but changes in the x-ray machine itself in the period of time between radiographs will change the quantity of radiation at the same machine setting and make almost impossible the task of exposing the patient to the same amount of radiation. Further, the weight of the patient may change in the interval between radiographs or the position or alignment ofthe patient on the table may change in taking the radiographs with the result that the total amount of radiation energy received by the photographic emulsions vary considerably in the taking of the two radiographs. Further, the commercial photographic film will change in sensitivity during the interim between the radiographs or the commercially available developer may change with the result that images of the same tissue on radiographs taken several months apart may vary considerably in density and make detection of growth of tumors by comparison of the radiographs alone almost impossible.

Changes in the amount of radiation delivered by the x-ray machine do not affect or hinder a comparison of radiographs in accordance with my invention. Further, diagnosis of tumor growth by comparison of radiographs taken months apart is no affected by and is independent of changes in weight of the patient; and still further, diagnosis utilizing my invention is not affected by, and is independent of, changes in total energy delivered by the x-ray machine, of changes in the absolute sensitivity of the film, and of changes in the developer in the interim between the radiographs providing the film characteristic curves are similar.

In accordance with my invention, a first roentgenographic emulsion 1 is exposed to radiation from x-ray machine 2 simultaneously passing through the subject 4 to be studied and calibrating wedge 30 of constant attenuating gradient illustrated in FIG. 6. The roentgenographic film 1 is developed, the developed film 10 is placed in contact with the emulsion side of a negative photocopy paper 11 and exposed to uniform intense light from source 16 in photocopy machine 14, and then processed in accordance with the steps described in detail above. The image on the positive 17 will have all densities below a predetermined density eliminated and will contain an outline of any tumor the subject may have. Further, the edge of the outline will be sharp. It will be appreciated that it may be necessary to repeat the procedure a number of times, while varying the exposure time of negative photocopy paper 11 in photocopy machine 14, to obtain a reproduction with the threshold density which will best accentuate the slight differences in optical density lines that portray the tumor in the subject. Positive [7 will also have a reproduction 32 of wedge 30 (See FIG. 1).

It it is desired to determine whether the tumor has grown over a period of several months, a second roentgenographic emulsion l is subsequently exposed to radiation from x-ray machine 2 simultaneously passing through the subject 4 to be studied and wedge 30. The second roentgenographic emulsion l is developed, the developed film l0 is'placed in contact with a second negative photocopy paper 11 and exposed to the bright intensity light from source 16 in photocopy machine 14 passing through the developed radiograph l0, and the exposure time of second negative photocopy paper 11 is controlled in photocopy machine 14 so that the image 32 of wedge 30 in the final positive paper is substantially identical to that of the image 32 in the reproduction of the initial radiograph, or in other words so that the change of density with distance in the images 32 on the two positive reproduction is the same. It will be appreciated that it may be necessary to expose several second negative photocopy papers 11 in photocopy machine 14 through the second radiograph with varying exposure times until the images 32 of the wedge 30 in the first and second reproductions 17 appear substantially alike.

The exposed second negative is then placed in contact with a positive photocopy paper 17, the negative and positive papers 11 and 17 so held in contact are passed through a developer solution 18 and separated a predetermined time after they are removed from the developer. Inasmuch as the reproductions 32 of the wedge 30 on the pictures taken several months apart have the same density gradient, equal densities in the tissue being studied will have the same appearance and tone on the two reproductions made months apart regardless and independent of changes in quantity of radiation at a given setting of the x-ray machine, of changes in weight or breathing of the subject, or of changes in the developer or in sensitivity of the film in the interim. Consequently, the images of the tumor on the two reproductions made months apart can be directly compared to determine if the abnormal tissue has grown in the interim. it is important that the characteristic curves of the two films are alike for quantitative comparisons to be made independent of controlled densities.

While only a few ways of practicing the invention have been illustrated and described, many modifications and variations thereof will be readily apparent to those skilled in the art; and consequently, it is intended in the appended claims to cover all such modifications and variations which are in the true spirit and scope of the invention.

lclaim:

l. The method of diagnosing abnormal tissue growth in a subject comprising the steps of exposing a first photographic film to radiation passing through said subject to be diagnosed, developing said first film, whereby the optical density pattern in said first film is a representation of said abnormal tissue growth, exposing a first negative photocopy emulsion to high intensity diffuse light of at least 800 foot candles passing through said developed first film, placing a first positive photocopy emulsion in contact with said exposed first nega' tive emulsion, inserting said first positive and first negative so held in contact in a developing solution, removing said first positive and first negative emulsions from said solution and separating them after a predetermined time, whereby the image on said first positive is an isodensity outline of said abnormal tissue growth.

2. The method in accordance with claim l wherein said radiation exposing step includes exposing said first photographic film to said radiation passing simultaneously through said subject and a calibrating object of predetermined attenuating gradient, and subsequently repeating all of said radiation exposing, developing, light exposing, placing, inserting, removing and separating steps with a second photographic film, a second negative photocopy emulsion, and a second positive photocopy emulsion to produce on said second positive an isodensity outline of said subject and including the step of controlling the time that the second negative photocopy emulsion is exposed to said high intensity diffuse light so that the reproduction of said calibrating object on said second positive is substantially identical to that on said first positive, whereby the difference in the images on said first and second positives is indicative of the growth of said tissue in the time interval between said first and subsequent radiation exposing steps.

3. The method in accordance with claim 1 and including the step of selecting the optical density to be erased on said positive by exposing said negative emulsion to said diffuse light for a predetermined time interval.

4. The method of diagnosing abnormal tissue growth in a subject comprising the steps of exposing photographic film to radiation passing through the subject to be diagnosed, developing said film, whereby the optical density pattern on said film is a representation of said abnormal growth, exposing a negative photocopy emulsion to high intensity diffuse light of at least 800 foot candles passing through said developed film, placing a transparent member having a positive photographic emulsion thereon in contact with said exposed negative photocopy emulsion and inserting them in a developer solution, and removing said transparent member and said negative photocopy paper so held in contact from said solution, whereby the image on said positive emulsion is an isodensity outline of said tissue growth visible through said transparent member.

5. The method in accordance with claim 4 including the step of selecting the optical density to be erased on said positive by exposing said negative emulsion to said diffuse light for a predetermined interval of time.

6. The method of diagnosing abnormal tissue growth in a subject comprising the steps of exposing a first roentgenographic film to radiation passing through the subject to be diagnosed and calibrating object of constant attenuating gradient, developing said film, whereby the optical density pattern on said film is a representation of said abnormal tissue growth, exposing a negative photocopy emulsion to high intensity diffuse light of at least 800 foot candles passing through said developed film, placing a positive emulsion in contact with said exposed negative emulsion and inserting them in a developing solution, whereby the image left on said positive emulsion is an isodensity outline of said tissue growth, subsequently exposing a second roentgenographic film to radiation passing through saidsubject to be diagnosed and said calibrating object, developing said second film, exposing a second negative photocopy emulsion to high intensity diffuse light of at least 800 foot candles intensity passing through said developed second film, placing a second positive emulsion in contact with said second negative emulsion and inserting them in a developing solution, and controlling the amount of time said second negative paper is exposed to said diffuse light so that the reproduction of said calibrating object on said second positive emulsion is substantially identical to that on said first positive emulsion whereby the difference in the image on said first and second positive emulsions is indicative of growth of said tissue in the time interval between said first and second radiation exposure steps and the diagnosing method is independent of absolute density control.

7. The method of obtaining isodensity lines from a photographic emulsion comprising the steps of: exposing a negative photocopy emulsion to high intensity diffuse light of at least 800 foot candles passing through said photographic emulsion,

placing a positive photocopy emulsion in contact with said exposed negative emulsion, inserting said positive and negative emulsions so held in contact in a developer solution, and removing said positive and negative emulsions from said solution and separating them after a predetermined time interval, whereby all optical densities below a given density are erased from said positive and the image thereon is an isodensity outline of said photographic emulsion.

8. The method in accordance with claim 7 and including the step of controlling the interval of time said negative emulsion is exposed to said diffuse light to select the optical density below which all densities are erased.

9. The method of obtaining isodensity lines from a photographic film comprising the steps of exposing a negative photocopy emulsion to high intensity diffuse light of at least 800 foot candles passing through said photographic film, placing a transparent member having a positive emulsion thereon in contact with said exposed negative emulsion, inserting said negative emulsion and transparent member so held in contact in a developer solution, and' removing said negative and transparent member from said solution, whereby the image on said positive emulsion is an isodensity outline of the image on said photographic film visible through said transparent member.

10. The method of diagnosing an object having differences in density which are not visually discernable comprising the steps of preparing a thin slice of said object, exposing a negative photocopy emulsion to high intensity diffuse light of at least 800 foot candles passing through said slice, placing a positive emulsion in contact with said exposed negative emulsion, inserting said negative and positive emulsions so held in contact in a developer solution, and removing said negative and positive emulsions from said solution andseparating them after a predetermined interval, whereby the image left on said positive emulsion is an isodensity outline of the structure of said object. 

2. The method in accordance with claim 1 wherein said radiation exposing step includes exposing said first photographic film to said radiation passing simultaneously through said subject and a calibrating object of predetermined attenuating gradient, and subsequently repeating all of said radiation exposing, developing, light exposing, placing, inserting, removing and separating steps with a second photographic film, a second negative photocopy emulsion, and a second positive photocopy emulsion to produce on said second positive an isodensity outline of said subject and including the step of controlling the time that the second negative photocopy emulsion is exposed to said high intensity diffuse light so that the reproduction of said calibrating object on said second positive is substantially identical to that on said first positive, whereby the difference in the images on said first and second positives is indicative of the growth of said tissue in the time interval between said first and subsequent radiation exposing steps.
 3. The method in accordance with claim 1 and including the step of selecting the optical density to be erased on said positive by exposing said negative emulsion to said diffuse light for a predetermined time interval.
 4. The method of diagnosing abnormal tissue growth in a subject comprising the steps of exposing photographic film to radiation passing through the subject to be diagnosed, developing said film, whereby the optical density pattern on said film is a representation of said abnormal growth, exposing a negative photocopy emulsion to high intensity diffuse light of at least 800 foot candles passing through said developed film, placing a transparent member having a positive photographic emulsion thereon in contact with said exposed negative Photocopy emulsion and inserting them in a developer solution, and removing said transparent member and said negative photocopy paper so held in contact from said solution, whereby the image on said positive emulsion is an isodensity outline of said tissue growth visible through said transparent member.
 5. The method in accordance with claim 4 including the step of selecting the optical density to be erased on said positive by exposing said negative emulsion to said diffuse light for a predetermined interval of time.
 6. The method of diagnosing abnormal tissue growth in a subject comprising the steps of exposing a first roentgenographic film to radiation passing through the subject to be diagnosed and calibrating object of constant attenuating gradient, developing said film, whereby the optical density pattern on said film is a representation of said abnormal tissue growth, exposing a negative photocopy emulsion to high intensity diffuse light of at least 800 foot candles passing through said developed film, placing a positive emulsion in contact with said exposed negative emulsion and inserting them in a developing solution, whereby the image left on said positive emulsion is an isodensity outline of said tissue growth, subsequently exposing a second roentgenographic film to radiation passing through said subject to be diagnosed and said calibrating object, developing said second film, exposing a second negative photocopy emulsion to high intensity diffuse light of at least 800 foot candles intensity passing through said developed second film, placing a second positive emulsion in contact with said second negative emulsion and inserting them in a developing solution, and controlling the amount of time said second negative paper is exposed to said diffuse light so that the reproduction of said calibrating object on said second positive emulsion is substantially identical to that on said first positive emulsion whereby the difference in the image on said first and second positive emulsions is indicative of growth of said tissue in the time interval between said first and second radiation exposure steps and the diagnosing method is independent of absolute density control.
 7. The method of obtaining isodensity lines from a photographic emulsion comprising the steps of: exposing a negative photocopy emulsion to high intensity diffuse light of at least 800 foot candles passing through said photographic emulsion, placing a positive photocopy emulsion in contact with said exposed negative emulsion, inserting said positive and negative emulsions so held in contact in a developer solution, and removing said positive and negative emulsions from said solution and separating them after a predetermined time interval, whereby all optical densities below a given density are erased from said positive and the image thereon is an isodensity outline of said photographic emulsion.
 8. The method in accordance with claim 7 and including the step of controlling the interval of time said negative emulsion is exposed to said diffuse light to select the optical density below which all densities are erased.
 9. The method of obtaining isodensity lines from a photographic film comprising the steps of exposing a negative photocopy emulsion to high intensity diffuse light of at least 800 foot candles passing through said photographic film, placing a transparent member having a positive emulsion thereon in contact with said exposed negative emulsion, inserting said negative emulsion and transparent member so held in contact in a developer solution, and removing said negative and transparent member from said solution, whereby the image on said positive emulsion is an isodensity outline of the image on said photographic film visible through said transparent member.
 10. The method of diagnosing an object having differences in density which are not visually discernable comprising the steps of preparing a thin slice of said object, exposing a negative photocopy emulsion to higH intensity diffuse light of at least 800 foot candles passing through said slice, placing a positive emulsion in contact with said exposed negative emulsion, inserting said negative and positive emulsions so held in contact in a developer solution, and removing said negative and positive emulsions from said solution and separating them after a predetermined interval, whereby the image left on said positive emulsion is an isodensity outline of the structure of said object. 